Less of a seal at the touchhole on a flintlock, more likely to have air get in. Spent percussion cap on the nipple prevents air from getting in. IIRC it was a on again of again serious problem for the Brits during the Napoleanic Wars, part of the reason they stressed the drill the way they did. And an issue for the US soldiers until Von Stueben started his training at Valley Forge.

Apparently only a serious enough problem to worry about during rapid fire.